(1) Field of the Invention
The subject of the invention is a method and a circuit for acoustic echo cancellation in VoIP terminal. The solution is intended for various types of client terminals of Internet voice communication systems, especially when the client uses a loudspeaker instead of a headset.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A development of voice transmission technologies that make use of computer networks, described using a term “Voice over IP (VoIP) telephony” is a source of numerous novel hardware and software solutions for effective transmission of the speech signal in the Internet, providing good quality of the signal. The clients of the VoIP systems are encouraged to use either a dedicated terminal, resembling the traditional telephone apparatus, or a headset consisting of headphones and a microphone. In some cases, because of various reasons, the clients need to use loudspeakers which often leads to decreased quality of the conversation due to the acoustic echo effect. The acoustic echo occurs if the far-end speech signal from the loudspeaker is collected by the microphone that should only record the near-end speech. As a result, the speech signal returns to the original speaker who hears his own voice, delayed and distorted, because the microphone at the client terminal collects not only the near speech, but also the undesired, distorted echo signal. In order to eliminate this effect, various signal processing algorithms and devices are used in order to prevent the echo signal from returning to the sender, without introducing considerable transmission delay, by means of removing the echo from the signal collected by the microphone and transmitting only the desired near-end speech signal.
A number of methods and devices for acoustic echo cancellation by means of an adaptive digital filtering is known. Processing the far-end speech signal by the adaptive filter results in obtaining an echo estimate which is then subtracted from the microphone signal. The result of this operation is used for filter adaptation. After the adaptation process is finished, the echo estimate from the adaptive filter output simulates the real acoustic echo and it may be subtracted from the microphone signal, resulting in echo cancellation. In order to achieve an accurate and efficient echo cancellation using the solutions based on adaptive filters, the filter adaptation process must not performed if a double-talk occurs, i.e. the microphone connected to the client terminal collects both near-end speech and, at the same time, echo signal from the far end. This requirement is necessary in order to prevent detuning of the filter and distortion of the processed signal. A number of methods and devices for double-talk detection, differing in complexity and accuracy, is known.
One known method of adaptive acoustic echo cancellation described in U.S. Pat. No. 4,894,820 uses the double-talk detection based on additional adaptive filter for estimation of the difference between the processed and the microphone signal, by means of comparing the statistical parameters of the signal. Another known solution described in U.S. Pat. No. 6,608,897 uses a variable filter adaptation step, depending on the difference between the processed signal, after echo removal, and the microphone signal. Yet another known double-talk detection method described in U.S. Pat. No. 6,792,107, suitable for implementation in the VoIP system, is based on calculation of a correlation between the far-end signal and the microphone signal. The correlation is a measure of similarity of the signals. If the correlation is low, a double-talk may be detected. Additionally, a dynamic threshold for the detection is determined from the analysis of both signals. Similarly, the invention described in U.S. Pat. No. 6,192,126 proposes double-talk detection by means of analysis of the signal energy in several frequency bands.
In the solution disclosed in U.S. Pat. No. 4,894,820, the acoustic echo canceller includes the digital adaptive filter and the so-called Geigel double-talk detector which compares the amplitude or energy of the microphone and the far-end signal. The international application WO98/43368 describes the device for echo cancellation containing the adaptive filter in parallel connection with the double-talk detector to which two nonlinear processors are connected, together with the noise generator, delay estimator, noise power estimator and two tone switches. Another acoustic echo canceller described in patent application WO98/51066, in addition to the adaptive filter, contains at least one additional microphone, another adaptive filter and the additional filters.
Known solutions for acoustic echo cancellation that provide high accuracy are based on complex algorithms. As a consequence, the double-talk detection requires more time and delays are introduced to the signal transmission. On the contrary, other solutions that do not introduce considerable delays, do not provide satisfactory accuracy of echo cancellation. Therefore, none of these solutions are optimal for application in VoIP terminals having limited resources.